Numerous publications, and in particular patents, already describe the control of DC motors, and in particular the control of low voltage DC motors of the kind used in components for motor cars. For example, French patent application No. 83 01748 describes clutch position control in which a moving component is displaced under the control of pulses at a recurrence frequency which is a function of the speed of the motor. French patent No. 1 369 512 relates to a system in which the rate and the width of the pulses may change as a function of an error voltage such as a speed error, thereby producing speed feedback.
However, the above-mentioned devices are nevertheless subject to the constraints which are specific to using DC motors of the kind generally used when providing position control, and these constraints are described below with reference to FIGS. 1 and 2, which show the conventional solutions generally employed with motors of this type. The electric motors generally used for this kind of position control have high torque and a high speed of rotation.
Electronic control for a servomotor is generally provided either in proportional mode or else in on/off mode.
In proportional mode as shown in FIGS. 1a and 1b, the voltage delivered to the motor 3 is inversely proportional to the error .epsilon. between the instantaneous position of the motor shaft (as given, for example, by a position-indicating system r) and the desired final position of the shaft as represented by a reference signal c. The voltage applied to the motor 3 is generated by means of a subtractor circuit 1, and where necessary, by means of an amplifier circuit 2. FIG. 1b shows that the voltage effectively applied to the motor falls off with travel of the motor shaft and ends at a threshold voltage marked V.sub.th.
Proportional mode control gives good accuracy in the position in which the shaft stops, and it also stops the shaft gently. However, it suffers from the drawbacks of poor efficiency due to the electrical power dissipated in the electronic circuits, and above all due to the existence of a permanent current flowing through the motor when it is in its stopped position, which current is due to the motor threshold voltage and gives rise to permanent dissipation in the control circuits. This phenomenon runs the risk of the equipment overheating and of protective means disconnecting the hot circuits.
In on/off control mode, as shown in FIGS. 2a and 2b, the reference or control voltage or signal c is applied to one of the inputs to the subtractor 1 and causes an error voltage .epsilon. to appear on the output thereof, which error voltage is suitable, in turn, for triggering a generator 4 which delivers a DC voltage directly to the motor. The voltage V.sub.motor applied to the motor then changes suddenly from maximum to zero when the instantaneous position of the motor shaft as represented by the position-indicating signal r approaches the desired position. FIG. 2b plots the voltage actually applied to the motor and the travel of its shaft as substantially represented by the signal r as a function of time.
Unlike proportional mode control, on/off mode control has the advantage of avoiding any risk of over-heating and consequent disconnection since no current flows through the motor when it is in its stop position. However the accuracy of the position in which the motor stops, and thus the accuracy of the position controlled thereby, is less good than in proportional mode and the sudden and staccato operation gives rise to a risk in position instability (often called "hunting") particularly when it is desired for the positioning error relative to the desired stop position to be compatible with the usual requirements in this kind of application.
Preferred embodiments of the present invention remedy the above drawbacks by providing a control method, a control device, and a system for controlling linear displacement of a DC motor, in which the drawbacks of proportional mode control and of on/off mode control are eliminated.